Manufacture of malleable cast iron



Patented Mar. 26, 1940 2,195,078 MANUFACTURE or MALLEABLE CAST mos Roysel J. Cowan, Toledo, Ohio, asslgnor to Surface Combustion Corporation, Toledo, Ohio, a corporation of New York No Drawing.

Application July 11, 1938,

Serial No. 218,641

1 Claim.

This invention relates to the art of heat treating white iron castings to produce malleable cast iron and has for its object to provide an improved annealing procedure whereby to produce 5 an improved product of malleable cast iron.

The' art of making malleable iron is one of the old and well established operations of iron manufacture. It relates to a specific type of material widely used incommerce. The form in which it is used as an article of commerce is quite different from the form in which it is at first made. .When. first prepared in the form: of castings, it is known as white cast iron or hard iron because of the fact that it is indeed extremely hard and brittle. When these castings are broken as by a sharp blow with a hammer,

the fractured section is found to be white in appearance. This is due to the fact that all of the carbon contained in the material is in the mbined form which is known to produce a s eely appearance. At this stage in the manufacture it is the custom to insure a complete absence of graphitic carbon in the structure. In fact,

it is a part of the art of manufacture to adjust the chemical composition of the iron so as to produce a hard iron casting free from graphite.

The hard iron casting so produced is subjected to a heat-treating operation frequently called annealing but more properlygraphltizing.

The purpose of this heat-treatment is to bring about a chemical and physical change in the carbon contained within the material and thus change it into one that is soft and malleable.

By this treatment the combined carbon of the hard iron which is present in the form of cementite having the formula FeaC is broken down into free carbon and iron, the carbon then existing in a special form of graphitic carbon known as temper graphite or tempercarbon.

4o Temper graphite is distinguished from the usual graphite that is to be found in cast iron by the fact that it is nodular in form and this nodular form especially characterizes the malleable iron used in industry. As a result of this graphitize- 45 tion process, the material is rendered ductile and malleable so that it may be bent and distorte considerably without fracturing.

The annealing or graphitizing operation as usually conducted consists of essentially two 50 parts commonly referred to as flrststage and second stage annealing which follow each other in succession. In first stage annealing, the hard iron is heated to a temperature above the critical point of the metal usually between 1600 F. and 1750" F. for a certain length of time-which depends upon the metal composition. After this, it is cooled to a temperature close to the critical point at whichit is held for another period of time, so .as to complete the second stage annealing. The length of time required for this 5 second stage annealing is dependent also upon the particular metal composition that is being heat-treated. First stage" therefore consists of a hold at high temperature, and, second stage" of a' drop in temperature to the vicinity of the 10 critical point and careful heat-treatment at this point, after which the metal may be cooled to room temperature in any manner desired.

As annealing operations are ordinarily conducted, it has been found that more or less de- 15 carburization occurs during the first stage heattreatment of white cast iron. This decarburization extends some distance into the body of the casting, and frequently to a. depth of it". The

, decarburization of this area is not. complete to a the extent that all of the carbon has been removed, but removed only to a point which is much lower in value than that of the original hard iron. Experience has'shown that this has of the decarburized areas during second stage annealing in that it retards this rate very considerably. This is probably due to the fact that it requires aAmuch longer time to graphitize a low carbon material than to graphitize a high 30 carbon material. a As a. result, therefore, of the surface decarburlzation of hard iron first stage annealing, there does not occur, especially in the second stage annealing, a uniform graphitization throughout the entire cross section 5 of the casting being higher in carbon, for which reason the interior of the casting will be thoroughly graphitized while the edge will be incompletely so. This gives rise to what is widely known in the art as a white rim, some- 40 tim'es called a pearlitic edge. This incompletely graphitized rim is hard because it retains considerable combined carbon and is consequently very dimcult to machine. The easy machineability of malleable cast iron is a strong factor in support of its-industrial use. It is obvious,

therefore, that any surface condition that-may be brought about during heat treatment which will adversely affect machineability is very objectionable. In ordinary malleableizing prac-' tice in order to insure the absence of white rims on the castings it'is customary to prolong the graphitize'the core of the casting. The neces- 0/;

/ iron frequently called ferrite.

sity for this may be traced back to the decarburization which occurs during the early part of first stage annealing. Furthermore, this prolonged holding time removes the combined carbon from the surface of the casting by oxidization thereby producing a carbon-free area of This ferritic edge is free from temper graphite and is, consequently, difficult to machine, since ferrite, being very soft, has a-strong tendency to tear and to clog the tool rather than to cut smoothly. Malleable cast iron, as found today in commerce, has either a pearlitic or a ferritic edge, and is characterized by the absence of temper graphite at the edges of the casting. 4

I have found, and this forms the basis of my invention, that it is possible to conduct the annealing of white cast iron so as to retard or absolutely' prevent decarburization during the graphitizing operation by the employment of special atmospheres during the successive annealing stages, I accomplish this by conducting the operation in a furnace structure designed to -ex-- clude air and retain within the work treating chamber an atmosphere of special composition.

In carrying out the operations of this invention, K

I do not use the sameatmosphere throughout the entire malleableizing cycle, but instead, I

vary the composition of the atmosphere which I flow into the chamber in accordance with the requirements of the process. Thus, while the metal is coming to temperature, and for a short time during first stage anneal, I use an atmosphere which tends to carburize the iron and which preferably consists of a mixture of products of combustion and fuel gas. Following this period I use a decarburizing gas which preferably consists of products of combustion or various mixtures of its components.

A typical time-temperature cycle employed in connection with the annealing of hard iron, and which may be used in connection with my process, is as follows:

(a) The heating period during which the metal is raised from room temperature to the maximum temperature which point is determined by the chemical composition of the material, its

weight or size.

(1)) A holding period at this temperature of sufficient duration to bring about first stage annealing.

(c) A, more or less rapid cooling from this temperature to a lower one in the vicinity of the critical point of the iron as commonly recognized.

(d) A holding or slow cooling period around the critical point of sufiicient duration to accomplish the completion of second stage annealing.

To furnish the products of combustion used in my process, I employ a gas generating unit capable of producing products of combustion of definite composition. These products of combustion are formed by means of the combustionof fuel gas and air mixtures in the proportions that will maintain combustion, it being understood that its composition may vary within rather wide limits, depending upon the proportions of air and fuel gas used. To'these products of combustion I add at a definite stage of the heat-treating cycle a regulated amount of a hydrocarbon gas such as natural gas, propane, butane, or manufactured gas commonly called city gas.

In conducting the annealing operation in accordance with my method I, first of all, purge all air from the chamber by means of products of combustion. At some point during the temperature rise of the iron I mix with the products of combustion a desired amount of fuel gas, preferably natural gas and maintain the fiow of this mixture of gases from this point on until the 5 maximum temperature has been reached and for an additional period of time varying usually from one to ten hours during the first stage annealing. At the end of this period the addition of the fuel gas to the products of combustion is discontinued 10 and throughout the remainder of the cycle products of combustion alone are used as the atmosphere.

The proportions of fuel gas and products of combustion that are used in my process are de- 1! combustion is determined also by the amount of 2! decarburizing that is to, be done during second stage annealing. During this period decarburization affects the combined carbon which is present in solution with the iron rather than the precipitated carbon that is present as temperfi graphite. Consequently a controlled amount of decarburization during this, period will assist in removing the carbon and thereby accelerating the annealing rate by this means as well as by graphitization. That is to say, the combined carbon in the iron may be removed from solution by oxidization as well as by precipitation. The total volume of gases required to keep the annealing chamber full has a bearing also on gas composition since a large fiow will require less car- 4| bon dioxide to be decarburizing than a lesser fiow.

I have observed that the employment for too long a period of time during first stage annealing of an atmosphere that tends to carburize the iron will have an effect upon the metal which will 4 retard the graphitizing reaction. I do not fully understand the physical or chemical reactions involved in this retardation but I have observed that the period'of time during which the said atmosphere can be used to advantage is influ- 6 enced by such factors as the chemical composition of the iron, the maximum temperature employed for first stage annealing, and the proportion of hydrocarbon gas in said atmos-.'

phere. However, after graphiti'zation has begun, 5

the combined carbon content of the hard iron is progressively reduced until it reaches a point where it is so low that carburization can occur in the said atmosphere. Such recarburization during the later stages of first stage annealing 6 is very objectionable and will retard the annealing rate. Because of this I discontinue the addi tion of hydrocarbon gas to the products 'of combustion before or at the time this phenomena takes place and carry out the remainder of the li annealing cycle in such a way as to prevent carburization even to the extent of using a decarburizing atmosphere.

Experience has shown that the employment during second stage annealing of an atmosphere 7 that tends to be decarburizing is not objectionable. The only limit as to how far the decarburizing characteristic of the atmosphere can be carried semes to be that point at which oxidization or the metal would occur as distin- 1 Such a K cross-section extending even to the extreme will produce very satisfactory results.

The method of heat treatment herein described has been employed in several commercial installations. The results obtained have been of superior character. Several beneficial effects have been noted. In the first place, there has been a reduction in time required for complete malleableization. time hasbeen reduced from six days to forty hours. Furthermore, the iron itself is of superior character due to the fact that temper car- .bon, or that particular type of graphitewhich is characteristic of malleable cast iron, is found uniformly distributed throughout the metal In these installations this combustion.

edges of the casting. Because of this, the metal so produced has unusual machining qualities that are due to the presence of temper graphite at the edge rather than a pearlitic structure at 5 the edge which is so frequently encountered in commercial malleables.

What I claim and desire to secure by Letters Patent is:

The process of annealing hard iron for the 10 production of malleable cast iron, which comprises heating the iron to the temperature of first stage annealing and during the heating-up stage and for substantially less than the full.

period of first stage annealing, subjecting the 15 iron to an atmosphere comprising hydrocarbon gas in substantial amounts effective to prevent metal decarburization, and thereafter during the remainder ofthe heat treatment and prior to second stage annealing, subjecting the iron to 20 an atmosphere comprising gaseous products of v ROYSEL J. COWAN. 

